Wavefront coding phase contrast imaging systems

Abstract

The present invention provides extended depth of field or focus to conventional Phase Contrast imaging systems. This is accomplished by including a Wavefront Coding mask in the system to apply phase variations to the wavefront transmitted by the Phase Object being imaged. The phase variations induced by the Wavefront Coding mask code the wavefront and cause the optical transfer function to remain essentially constant within some range away from the in-focus position. This provides a coded image at the detector. Post processing decodes this coded image, resulting in an in-focus image over an increased depth of field.

What is claimed is:  
     
       1. Apparatus for increasing depth of field and controlling focus related aberrations in a Phase Contrast Imaging system having an illumination source, illumination optics, and an illumination mask placed before a Phase Object to be imaged, and an objective mask and objective optics after the Phase Object to form an image at a detector, the improvement comprising: 
       an optical Wavefront Coding mask having an aperture and placed between the Phase Object and the detector,  
       said coding mask being constructed and arranged to alter the optical transfer function of the Phase Contrast Imaging system in such a way that the altered optical transfer function is substantially insensitive to the distance between the Phase Object and the objective optics over a greater range of object distances than was provided by the unaltered optical transfer function,  
       wherein the coding mask affects the alteration to the optical transfer function substantially by affecting the phase of light transmitted by the mask; and  
       a post processing element for processing the image captured by the detector by reversing the alteration of the optical transfer function accomplished by the coding mask.  
     
     
       2. The apparatus of  claim 1  wherein the detector is a charge coupled device (CCD). 
     
     
       3. The apparatus of  claim 1 , wherein the phase of light transmitted by the coding mask substantially follows a cubic function. 
     
     
       4. The apparatus of  claim 1 , wherein the phase of light transmitted by the coding mask is relatively flat near the center of the aperture with increasing and decreasing phase near respective ends of the aperture. 
     
     
       5. The apparatus of  claim 4 , wherein the phase of light transmitted by the coding mask is a smoothly varying function having peak to valley phase changes of about +5 radians to −5 radians. 
     
     
       6. The apparatus of  claim 4 , wherein the phase of light transmitted by the coding mask is a smoothly varying function having peak to valley phase changes of about +12 radians to −12 radians. 
     
     
       7. The apparatus of  claim 4 , wherein the phase of light transmitted by the coding mask substantially follows a sum of powers function of the form: 
       
         
           phase( x,y )=exp( j* [Σa   i  sign( x )| x|   b   i   +c   i sign( y )| y|   d   i ]),  
         
       
       where sum Σ is over the subscript i; and  
       where sign(x)=−1 for x<0, +1 for x≧0, and j=sqrt (−1).  
     
     
       8. The apparatus of  claim 1 , wherein the phase of light transmitted by the coding mask substantially follows a non-separable function of the form: 
       
         
           phase( r ,θ)=exp( j* [Σr   a   i  cos( b   i θ+φ i )])  
         
       
       where sum Σ is over the subscript l, and j=sqrt (−1).  
     
     
       9. The apparatus of  claim 1 , wherein the coding mask further comprises a lens element for focussing the light. 
     
     
       10. The apparatus of  claim 1 , wherein the coding mask is integrally formed with the objective mask. 
     
     
       11. The apparatus of  claim 10 , wherein the objective optics are further integrally formed with the coding mask and the objective mask. 
     
     
       12. The apparatus of  claim 1 , wherein the coding mask comprises an optical material having varying thickness. 
     
     
       13. The apparatus of  claim 1 , wherein the coding mask comprises an optical material having varying index of refraction. 
     
     
       14. The apparatus of  claim 1 , wherein the coding mask comprises spatial light modulators. 
     
     
       15. The apparatus of  claim 1 , wherein the coding mask comprises micro-mechanical mirrors. 
     
     
       16. The method for increasing depth of field and controlling focus related aberrations in a conventional Phase Contrast Imaging system having an illumination source, illumination optics, and an illumination mask placed before a Phase Object to be imaged, and an objective mask and objective optics after the Phase Object to form an image at a detector, the method comprising the steps of: 
       between the Phase Object and the detector, modifying the wavefront of transmitted light;  
       the wavefront modification step selected to alter the optical transfer function of the Phase Contrast Imaging system in such a way that the altered optical transfer function is substantially insensitive to the distance between the Phase Object and the objective optics over a greater range of object distances than was provided by the unaltered optical transfer function; and  
       post processing the image captured by the detector by reversing the alteration of the optical transfer function accomplished by the mask.  
     
     
       17. The method of  claim 16 , wherein the phase of light transmitted by the mask substantially follows a cubic function. 
     
     
       18. The method of  claim 16 , wherein the modifying step modifies the phase of light transmitted according to a profile which is relatively flat near the center of the aperture with increasing and decreasing phase near respective ends of the aperture. 
     
     
       19. The method of  claim 18 , wherein the phase of light transmitted by the mask substantially follows a sum of powers function of the form: 
       
         
           phase( x,y )=exp( j*[Σa   i sign( x )| x|   b   i   +c   i sign( y )| y|   d   l ]),  
         
       
       where sum Σ is over the subscript i; and  
       where Sign(x)=−1 for x<0, +1 for x≧0.  
     
     
       20. The method of  claim 16 , wherein the phase of light transmitted by the mask substantially follows a non-separable function of the form: 
       
         
           phase( r ,θ)=exp( j*[Σr   a   i  cos( b   i θ+φ i )])  
         
       
       where sum Σ is over the subscript i.  
     
     
       21. A Wavefront Coding Phase Contrast Imaging system for imaging a Phase Object comprising: 
       an illumination source;  
       illumination optics placed between the illumination source and the Phase Object;  
       an illumination mask placed between the illumination source and the Phase Object;  
       a detector;  
       an objective mask placed between the Phase Object and the detector;  
       objective optics placed between the Phase Object and the detector to form an image at the detector;  
       an optical Wavefront Coding mask having an aperture and placed between the Phase Object and the detector,  
       said mask being constructed and arranged to alter the optical transfer function of the Imaging system in such a way that the altered optical transfer function is substantially insensitive to the distance between the Phase Object and the objective optics over a greater range of object distances than was provided by the unaltered Imaging system optical transfer function,  
       wherein the mask affects the alteration to the optical transfer function substantially by affecting the phase of light transmitted by the mask; and  
       a post processing element for processing the image captured by the detector by reversing the alteration of the optical transfer function accomplished by the mask.  
     
     
       22. The apparatus of  claim 21 , wherein the phase of light transmitted by the coding mask is relatively flat near the center of the aperture with increasing and decreasing phase near respective ends of the aperture.